The important properties that molding sands for green sand molds should generally have are that by using the molding sands the green sand molds can be readily formed, and that molten metals having a high temperature can be satisfactorily poured into the green sand molds that have been prepared by using the molding sands without the molten metals being disadvantageously affected. The former is a property that is described as a moldability of molding sands at ordinary temperatures. On the other hand, the latter is a property of molding sands at high temperatures.
To make a mold, first a molding sand must be adapted to a pattern to exactly copy it. Then, after the mold is made, the pattern is drawn out from the mold. Also after the pattern is drawn out, the sand mold must keep its shape, and must resist the pressure, impact, etc. from a molten metal. To satisfy these requirements, the molding sand must have an excellent flowability, and ability for binding sand particles with each other. However, generally, the flowability of the molding sand and the property in which sand particles are bound together (i.e., the ability for binding) are incompatible.
Conventionally, to simultaneously obtain both properties that are generally incompatible, a high-quality clay has often been used that has high mold cramping force even with a low water content, and as an aggregate a silica sand, the particles of which are uniformly shaped and roundish, has often been used due to its having good flowability. However, the high-quality clay and the uniformly shaped and roundish silica sand particles have become increasingly expensive year by year due to the apprehension regarding the exhaustion of such resources. Therefore, the artisan has coped with this situation by making much of any one of the two properties that are incompatible, instead of using both the high-quality clay and the uniformly shaped and roundish silica sand particles. That is, based on the recognition that the flowability and the ability for binding that have been demanded for molding sands are generally incompatible, an artisan seeks the best condition of the molding sand depending on the use of the green sand mold by altering the strength of one of the properties. Further, for maintaining the conditions of the molding sand, many efforts are made by using the five senses to adjust the sand temperature, water content, temperature, humidity, etc. However, in many cases, artisans are not satisfied with the quality of the molding sand.
The present inventors have investigated clay and silica sand. As a result, they have recognized that there are abundant resources of clay and silica sand that can be used and inexpensively obtained, provided that the molding sands obtained from the clay and silica sand, the resources of which are abundantly present, can manifest satisfactory flowability.
Thus, the object of the present invention is to provide a molding sand having a suitable flowability necessary for giving a high moldability and an ability for binding necessary for giving a satisfactory mold strength after sand molds are made, and a process for preparing the molding sand.